Positron emission tomography (PET) is a technique used in clinical medicine and biomedical research to create images that show anatomical structures as well as how certain tissues are performing their physiological functions. Radioactive nuclei are introduced into the body as labels on tracer molecules. These nuclei emit positrons which collide with electrons in the tissue. Each collision constitutes an annihilation event that may result in two gamma photons. By detecting the gamma photons and processing the result with image processing tools, an image of the activity in the tissue can be produced to display the physiological functions.
In positron emission tomography (PET) systems, a scintillation light pulse generated upon the interaction of a photon from the annihilation event with a scintillator is collected by photomultiplier tubes (PMT), or avalanche photodiodes (APD), and converted into a charge pulse. Hamamatsu Photonics K.K. Electron Tube Center, Fundamental and Applications of Photomultiplier Tube, JP:Hamamatsu Photonics K.K, 1995, the contents of which are hereby incorporated by reference, provides further details on the PMT. The charge pulse is often amplified and filtered to form a new voltage pulse that has a peak amplitude proportional to the area under the original scintillation light pulse, and hence proportional to the amount of photon energy that is deposited in the scintillator during the interaction. The peak amplitude is then sampled and converted into digital data by use of analog-to-digital converters (ADCs) for subsequent processing. An event time is typically obtained by using constant fraction discriminators (CFDs). Depth of Interaction Detector Block for High Resolution Positron Emission Tomography (U.S. Pat. No. 6,288,399 to Andreaco et al.), provides further details on implementation of PET detectors.
Efforts to achieve higher spatial resolution and a larger imaging volume have led to use of more and more small scintillators in PET design. Since every scintillator output needs to be separately processed, the number of ADC channels in a modern PET system is rapidly increasing. In addition, as faster scintillators and a 3D imaging mode are more widely used, high-speed ADCs are often desirable. However, a PET system that employs a large number of high-speed ADCs not only consumes a large amount of power, but also is often too expensive for many applications.